This invention relates to mechanical fastening means in general, and in particular to torque limiting fasteners.
The use of controlled torque in fastening various assemblies has widespread use, particularly in applications having close tolerances, or in applications where the the control of the mating surfaces of the final assembly is critical to prevention of excessive stresses and maintenance of required specifications over the temperature ranges to which the final assembly is subjected.
In general, this torque control is accomplished with the use of a limited torque applying tool such as a torque wrench, which has the ability to input a preset torque into the wrench, or which has a torque gauge installed thereon for observing the torque during installation of the threaded fastener.
In some applications, however, the expense of a torque limiting tool, or the close working conditions under which the fastener is to be installed, do not permit the use of a torque wrench. Thus, the assembly being fastened may be subject to excessive torque and, therefore, damage.
Additionally, some applications require the repeated installation and removal of the fasteners under various and often unpredictable circumstances; a torque limiting tool may not always be available. These circumstances become critical when the fastener is utilized to install very expensive components, as for example avionics equipment costing thousands of dollars. If the fastener should be over-torqued, the resultant damage could convert a 20 minute instrument exchange into a situation requiring complete replacement of a very expensive instrument.
Alternatively, a race car driver may desire to quickly change the compression ratio of the engine in a race car, which requires a precise control of the torque holding the engine head to the block. Under the limited time restraints often experienced under race conditions, the procedures utilizing a torque wrench must be closely observed to prevent damage to the engine. An additional problem arises when various fasteners for the engine head have differing torque values; the mechanic must use either a plurality of preset torque wrenches or observe the indicator on a single torque wrench while inserting the plurality of fasteners with varying torque.
In the examples above, it can be seen that the risk of damage to expensive equipment through improperly controlling maximum torque can be a problem, and the risk is increased when limited time is allowed for the insertion of the fastener to be controlled, or where the fastener will be repeatedly inserted under essentially uncontrolled conditions.